In an example of a prior art vehicle surroundings monitoring apparatus for monitoring the surroundings of a vehicle, an ultrasonic sensor is attached at a corner of the front or the rear of the vehicle so that a surrounding obstacle is detected or its distance is measured. Then, in case of danger of contact, the situation is reported to the driver (see, for example, Japanese Patent Publication No. 3232162).
FIG. 27 is a plan view showing a prior art vehicle surroundings monitoring apparatus described in Japanese Patent Publication No. 3232162. In FIG. 27, ultrasonic detectors 3FR, 3FL, 3RR, 3RL, 3BR, and 3BL are installed at the corners of the front or the rear of a vehicle body 1. Ultrasonic waves are transmitted in such timing that mutual interference is avoided. By virtue of this, a surrounding obstacle is detected, so that the distance to the obstacle is measured. Then, when the distance to the obstacle is reduced so that danger is expected, an alarm signal is outputted from an alarm unit.
Here, such ultrasonic sensors have a short detection distance and poor environmental resistance such as erroneous detection caused when raindrops are attached to the sensors. Thus, radar sensors employing radio waves also have begun to be adopted widely in vehicle surroundings monitoring apparatuses. A prior art radar apparatus for vehicle is installed in the front of a vehicle and used for measuring the car-to-car distance and the relative velocity to a vehicle located ahead by using an extremely narrow beam of 10 degrees or less, and thereby controlling the own vehicle. Thus, the monitoring area is limited to an extremely narrow and elongate region in front of the vehicle.